Examination of 3,400 hearts during the last 8 years disclosed at least 8 to have hypoplasia of both right and left circumflex coronary arteries. The 8 patients ranged in age from 26 to 79 years (mean 46). In 6 of the 8 patients the left anterior descending coronary artery coursed past the left ventricular apex to ascend on the posterior surface of the heart for distances ranging from 2 to 5 cm. The right and left circumflex coronary arteries were small and neither coursed to the crux posteriorly. The right coronary artery remained in the right atrioventricular sulcus for distances varying from 6 to 11 cm. These distances, compared to the total potential lengths of the right coronary artery from its ostium in the aorta to the crux posteriorly, indicate that this artery remained in the atrioventricular sulcus for 50 to 85% of the potential distance within the atrioventricular sulcus from its aortic origin to the crux posteriorly. The left circumflex coronary artery remained in the left atrioventricular sulcus for distances varying from 3 to 9 cm, numbers indicating that the left circumflex coronary artery was located within the left atrioventricular sulcus for 27 to 75% (mean 54%) of the total potential length of the left atrioventricular sulcus from the origin of the left circumflex from the left main coronary artery to the crux posteriorly. In summary, the hearts from 8 patients are described with hypoplasia of both right and left circumflex coronary arteries. It appeared that 2 of the 8 had evidence of myocardial ischemia as a direct consequence of the bilateral hypoplasia.